Understanding Your Eye Prescription

The sphere value, listed as SPH, is the most important number on your prescription. It measures the lens power needed to correct nearsightedness or farsightedness, expressed in units called diopters. A negative SPH means you are nearsighted, meaning distant objects appear blurry without correction. A positive SPH means you are farsighted, meaning close-up tasks can be difficult.

Prescriptions are written in 0.25-diopter increments. A prescription of -2.00 requires a stronger lens than -1.00. If your SPH is close to zero, such as -0.25 or +0.25, your correction is mild and you may not need glasses for every activity.

The cylinder value (CYL) and axis value appear together and are used to correct astigmatism, a condition where the cornea or lens has an uneven curvature that causes blurry or distorted vision. CYL measures how much astigmatism correction you need. The axis identifies the orientation of that correction on a scale of 0 to 180 degrees.

If a CYL value appears on your prescription, an axis value must also be present. If no CYL is listed, you do not have significant astigmatism that requires correction.

The ADD value appears on prescriptions for patients who need extra help with close-up tasks such as reading or using a phone. This value is related to presbyopia, which is the age-related gradual loss of the ability to focus up close. Presbyopia typically begins around age 40 and progresses over time as the natural lens inside the eye loses flexibility.

A typical ADD value ranges from +0.75 to +3.00 and is usually the same for both eyes. Your eye doctor adds this power to your distance prescription to create bifocal lenses, progressive lenses, or dedicated reading glasses. If your prescription has no ADD value, your near focus is still strong enough without additional correction.

Pupillary distance, or PD, is the measurement in millimeters between the centers of your two pupils. This number ensures your lenses are optically centered so that you look through the clearest part of each lens when wearing your glasses. The typical PD range for adults falls between 58 and 68 millimeters.

Some prescriptions list a single binocular PD, while others list separate measurements for each eye, called monocular PD. Monocular measurements are more precise and are especially important for progressive lenses. If your PD is not listed on your prescription, ask your eye doctor or optical shop to measure it before ordering glasses.

No, and attempting to do so may result in lenses that do not fit or correct your vision properly. Contact lens prescriptions include base curve and diameter values that do not appear on eyeglass prescriptions, and the power values may also differ. A separate contact lens fitting appointment and an updated contact lens prescription are required before ordering contacts from any retailer.

This is completely normal. Each eye develops its own shape and focusing characteristics independently, so different prescriptions in each eye are common. In some cases, the difference can be significant, a condition called anisometropia. Your eye doctor corrects each eye individually so that both work together comfortably for clear binocular vision.

Eyeglass prescriptions are typically valid for one to two years depending on your state. Contact lens prescriptions generally expire after one year. After expiration, a new comprehensive eye exam is needed to confirm your prescription still matches your current needs. While wearing a slightly outdated prescription will not damage your eyes, your vision may not be as clear or comfortable as it could be with an updated correction.

A short adjustment period of one to two weeks is common with a new prescription, particularly for first-time progressive lens wearers. However, if headaches persist beyond two weeks, or if your vision feels worse than with your old glasses, contact your eye doctor promptly. The issue may be with the prescription values, the lens measurements, or how the frames are fitting on your face, all of which can be corrected.

Not always. Some prescriptions include PD and others do not, depending on the practice. If your prescription does not list a PD, your eye doctor's office or an optical shop can measure it for you. An accurate PD is especially important for progressive lenses, where even a small centering error can cause visual discomfort. While some online tools offer self-measurement options, a professional measurement is always more reliable.

Yes, certain health conditions can affect your vision and shift your prescription. Uncontrolled blood sugar in diabetes can temporarily alter how your lens bends light, causing prescription fluctuations. Cataracts can gradually change your prescription over time. Hormonal changes related to pregnancy or menopause can also cause temporary shifts. If you notice a sudden or significant change in your vision between exams, it is worth scheduling an appointment sooner rather than waiting for your next routine visit.